An inlet for a supersonic engine must slow the airstream speed from supersonic speeds to the required subsonic speed for the engine. Hypersonic aircraft currently being designed have top operating speeds in the range of Mach 6 to Mach 8 and higher relative to the free airstream. In contrast, the required airstream speed at the aircraft engine intake may be as low as Mach 0.15 for a ramjet at these flight speeds. The shape of the inlet slows the airstream from the supersonic free stream speed to the engine intake speed.
Supersonic inlets fall into two broad categories, external compression and mixed compression inlets. In an external compression inlet, the normal shock wave, i.e., the region in which the airstream transitions from supersonic speed to subsonic speed, is located external of the cowl at approximately the cowl lip of the inlet. The cowl lip aperture is also the geometric throat of the inlet. An external compression inlet is illustrated in U.S. Pat. No. 3,450,141, to Braendlein, incorporated herein by reference. External compression inlets are presently used on most supersonic aircraft, such as the F-14, F-15 and F-18. One advantage of external compression inlets is their stability over a large range of operating conditions. One disadvantage of an external compression inlet is the excessive drag created by the inlet at high Mach numbers.
In a mixed compression inlet, the normal shock wave is located within the inlet duct, aft of the cowl lip. The airstream is slowed from a first supersonic speed to a second, lower supersonic speed by ramps external to the cowl lip. The airstream is still at a supersonic speed when it enters the inlet duct. The geometric throat of the inlet is located aft of the cowl lip, within the inlet duct. Mixed compression inlets are illustrated in U.S. Pat. No. 4,307,743, to Dunn, and British Patent No. 1,438,769, to Brooks, incorporated herein by reference. Mixed compression inlets are presently used on the SR-71, a twin-engine supersonic aircraft.
Mixed compression inlets have the advantage of being more aerodynamically efficient than external compression inlets at high Mach numbers, but also have some disadvantages. A mixed compression inlet has a very narrow range of stable operation. Mixed compression inlets are subject to "unstart," a phenomena in which the normal shock wave moves from the inlet duct to the cowl lip, causing the inlet to function as an external compression inlet. When a mixed compression inlet unstarts, the thrust produced by the engine is significantly decreased and, at the same time, the drag caused by the inlet is significantly increased. The simultaneous decrease in thrust and increase in drag have a drastic effect on the aircraft's flight characteristics. If the aircraft is traveling at high speeds when unstart occurs, the aircraft will rapidly slow down, making control of the aircraft by the pilot difficult or impossible for a period of time. In the SR-71, which has two engines with corresponding inlets, there have been reported examples of one of the inlets unstarting during supersonic flight, causing the aircraft to suddenly spin out of control. Unless the inlet is quickly "restarted," that is, the normal shock is moved from the cowl lip back into the inlet at the geometric throat, the pilot will not be able to regain control of the aircraft.